Growth of Petunia As Affected by Substrate Moisture Content and Fertilizer Rate

The rising costs of farm inputs, including fertilizer, have increased the cost of producing plants in the recent years.  Increasing water scarcity and rising concerns over environmental impacts of excessive water and fertilizer use in the horticultural industry are posing additional challenges to the industry.  In greenhouse production, much of the applied fertilizer can be lost through leaching if irrigation is excessive.  Soil moisture sensor-controlled irrigation can greatly reduce or even eliminate leaching, thus allowing for lower fertilizer rates.  The objective of our study was to determine the optimal fertilizer rates for petunia grown at different substrate volumetric water contents (θ). Petunias (Petunia × hybrida ‘Dreams White’) were grown at four substrate θ levels (0.1, 0.2, 0.3, and 0.4 m³·m^-3) and with eight fertilizer rates of controlled release fertilizer (Osmocote 14-14-14; 14N-6.1P-11.6K) applied at rates ranging from 0 to 2.5 g/plant.  Shoot dry weight increased with increasing fertilizer rates up to 1.3 - 1.7 g/plant, irrespective of θ. Shoot dry weight also increased with increasing substrate θ and plants were more response to fertilizer at higher θ (i.e. fertilizer x θ interaction). Substrate water content and fertilizer rate also affected leaf size; the size of leaves doubled as the  θ set point increased from 0.10 to 0.40 m³·m^-3  and increased by 16 – 34% as the fertilizer rate increased from 0 to 2.5 g/plant. Water use varied with the θ set point; approximately 0.2, 0.3, and 0.4 L/plant was needed to grow petunias from plug seedling to full bloom in 23 days at θ of 0.2, 0.3, 0.4 m³·m^-3, respectively.